Synthetic membranes and separating processes based on them have been known for a long time. In addition to classical areas of application, for example desalination of seawater by reverse osmosis or ultrafiltration of processing water from electrodip painting to recover paint, membrane processes are becoming increasingly important in the areas of food technology, medicine, and pharmaceuticals. In the latter cases, membrane separating processes have the great advantage that the materials to be separated are not subjected to thermal stress or even damaged.
An important prerequisite for the applicability of membranes in these areas is often that the membranes can be sterilized. For reasons related to safety and ecology, among others, steam sterilization is preferred over chemical sterilization (e.g., using ethylene oxide, or radiation sterilization, especially by gamma irradiation).
Steam sterilization is normally performed by treating the membrane or membrane system for approximately 1/2 hour with hot steam at temperatures greater than 110.degree. C. The criterion of steam sterilizability therefore limits the number of potential membrane materials considerably. Thus, for example membranes made of polyacrylonitrile basically cannot be steam-sterilized because exceeding the glass temperature of the polymer results in irreversible damage to the material or membrane. Hydrolysis-sensitive polymers (e.g., certain polycarbonates and polyamides) likewise cannot withstand hot steam sterilization undamaged.
Steam-sterilizable materials made of polyetherimides, polysulfones, or polyvinylidine fluoride are known. A major disadvantage of these membranes lies in the hydrophobic nature of the membrane material, which prevents spontaneous wetting with aqueous media. Consequently, the membrane must either be prevented from drying completely or it must be treated before drying with a hydrophilizing agent, glycerin for example.
Hydrophilic membranes are characterized by the fact that they are wettable with water. A measure of wettability is the edge angle which a water drop forms with respect to the surface of the membrane. In hydrophilic materials, this edge angle is always less than 90.degree.. The wetting of a dialysis membrane can also be detected by the fact that a drop of water applied to the surface of the membrane penetrates the membrane after a short time.
Another serious disadvantage of hydrophobic materials is that the materials often possess a powerful unspecific adsorption capability. Therefore, when hydrophobic membranes are used, rapid, firmly adhesive application of the membrane surface often takes place, preferably with higher-molecular-weight solvent components. This phenomenon, known as fouling, results in rapid deterioration of membrane permeability. Subsequent treatment of the membrane with a hydrophilizing agent cannot prevent fouling effectively.
Hydrophilic membranes which do not suffer from the above disadvantages have been proposed. For example, it is proposed in DE-OS 3,149,976 that a polymerizate mixture be used to manufacture a hydrophilic membrane, said mixture containing, in addition to polysulfone or polyamide, at least 15 weight percent polyvinylpyrrolidone. The use of polyethylene glycol in amounts of 44 to 70 weight percent, based on the polymer solution, is claimed in EP 0,228,072 for the hydrophilization of polyimide and polyether sulfone membranes.
However, the hydrophilization of membranes by the application of large quantities of water-soluble polymers has the disadvantage that the hydrophilicity of the membrane decreases steadily when it is used in aqueous media, since the water-soluble polymer is washed out. This can cause the membrane material to regain its original hydrophobic nature and exhibit the above-mentioned negative phenomena.
EP 0,261,734 describes the hydrophilization of polyetherimide membranes using polyvinylpyrrolidone. To prevent the washing-out effects, the polyvinylpyrrolidone is cross-linked in the non-swollen state. The method of manufacturing the membrane is very tedious, and therefore, cost-intensive, since the solvent and precipitant must be removed from the membrane after precipitation and before wetting, without removing the polyvinylpyrrolidone. It is only at this point that the polyvinylpyrrolidone is cross-linked by the application of high temperature or radiation, or chemically by means of isocyanates, whose residues absolutely must be removed completely before use when the membrane is to be employed in the food or medical area.
The disadvantages described above can be avoided by using hydrophilic, yet water-insoluble, polymers to make the membrane. Thus, in a number of patents, e.g. EP-0 182,506 and U.S. Pat. No. 3,855,122, manufacture of membranes from sulfonated polymers is claimed. However, the disclosed methods in these patents are only suitable for making flat membranes. They have a high salt retention capability and are used primarily for reverse osmosis.
U.S. Pat. No. 4,207,182 and two Japanese disclosure documents (JP-OS 61-249,504 and JP-OS 62-49,912) disclose another way to make hydrophilic membranes. According to these publications, hydrophilic membranes for ultrafiltration of aqueous solutions can be prepared advantageously from mixtures of sulfonated and non-sulfonated polysulfone.
The essential goal of the invention described in U.S. Pat. No. 4,207,182 is the use of highly concentrated polymer solutions to make membranes that are nevertheless characterized by high hydraulic permeability. This is accomplished by using polymer mixtures, with the quantity of sulfonated polysulfone based on the total polymer mixture of non-sulfonated and sulfonated polysulfone, being between 10 and 30 weight percent.
However, high hydraulic permeability is not advantageous for all applications. A high hydraulic permeability in dialysis results in back filtration and hence contamination of the liquid to be dialyzed by undesirable substances from the dialyzate.
As indicated by the examples in U.S. Pat. No. 4,207,182, the membranes according to the invention are also characterized by high screening coefficients for dextran with a molecular weight of 110,000 daltons.
Because of the high hydraulic permeability and the resulting high permeability to macromolecular substances with a molecular weight in excess of 100,000 daltons, the membranes that result from the claimed polymer mixtures are unsuitable for hemodialysis. This is all the more true when we consider that the dialytic permeability of the membranes manufactured in accordance with U.S. Pat. No. 4,207,182 is comparatively low.